1. Field of the Invention
The present invention relates to an internal combustion engine.
2. Description of the Related Art
It is well known to reduce the amount of production of NO.sub.x by recirculating part of the exhaust gas in an exhaust passage of an engine into an intake passage, i.e., to use EGR (exhaust gas recirculation).
On the other hand, Japanese Unexamined Patent Publication No. 63-131840 and No. 60-50241 disclose devices for controlling the amount of fuel to be injected. The devices can inject the proper amount of fuel into an intake port even when an engine is in a transient operating state, for example, an acceleration state or deceleration state. In this kind of the device, the basic amount of fuel to be injected is calculated on the basis of the engine speed and the pressure in the intake passage downstream of a throttle valve. When the engine is in a transient state, the difference between a successively detected first pressure and second pressure in the intake passage downstream of the throttle valve is calculated. The basic amount of fuel to be injected is corrected by a transient correction value in accordance with the difference.
However, when this device for controlling the amount of fuel to be injected is applied to an engine which has an EGR device, the following problem arises.
Referring to FIG. 4, when the engine is in an accelerating state, the pressure PM in the intake passage downstream of the throttle valve increases. During the acceleration, when the recirculation of the exhaust gas into the intake passage is started (at t1), the pressure PM rapidly increases due to the recirculation of the exhaust gas. However, as no fresh air is contained in the exhaust gas recirculated by the EGR device, the pressure PM does not exactly represent the amount of air fed into the engine cylinders, because the pressure PM contains an error corresponding to the pressure of the recirculated exhaust gas. Accordingly, when the transient correction value is calculated on the basis of the difference .DELTA.PM (=PM-PMO) between a first pressure PMO detected at t0 when the EGR operation is not carried out and a second pressure successively detected at t2 when the EGR operation is carried out, the transient correction value does not exactly represent the engine operating state and thus the air-fuel mixture fed into the engine cylinders becomes extremely rich.
Referring to FIG. 5, when the engine is decelerating, the pressure PM falls. During the deceleration, when the recirculation of the exhaust gas into the intake passage is stopped (at t4), the pressure PM rapidly falls. Accordingly, when the transient correction value is calculated on the basis of the difference .DELTA.PM between a third pressure detected at t3 when the EGR operation is carried out and a fourth pressure successively detected at t5 when the EGR operation is not carried out, the transient correction value does not exactly represent the engine operating state and thus the air-fuel mixture fed into the engine cylinders becomes extremely lean.
Namely, a problem occurs in that the air-fuel ratio differs considerably from the stoichiometric air-fuel ratio when the EGR operation is started or stopped during a transient engine operating state, for example acceleration or deceleration.
Also, due to this problem, a problem occurs in that the toxic components in the exhaust gas increase when the EGR operation is started or is stopped.